"Formal charge" games are just that, games. Ordinarily, as A-nuc has done, one follows a heirarchy of formal charges in which oxygen outranks every other species with a formal charge of -2. The part of the problem statement, "...in which all the bonds on chlorine are single bonds," implies an unnatural situation in which you are asked to calculate formal charge on Cl with -1 formal charge on the three oxygen atoms.

It's artificial, it's silly, and it looks to be something your instructor or text want you to do. There's absolutely no physical reason, quantum mechanical calculation, rationalization, or anything else to support such a picture. Nor, strictly, is there any reason to support a -2 state for oxygen in all cases. "Formal charge" is something you'll need to take seriously in courses that present molecular structures in terms of formal charge, but in the long run, the concept is useless beyond doing the bookkeeping on charge.